1. Field of the Invention
This invention relates to a signal transmitting/receiving method and apparatus, applied with advantage to a portable telephone system, a cordless telephone system and an inside radio communication system.
2. Related Art
Since the bandwidth of radio communication is finite, attempts have so far been made for developing a radio system having high frequency utilization efficiency. For this reason, a multiplication technique of synthesizing plural different items of the information and transmitting the resulting multiplexed signal has become widely known. Among the multiplexing techniques, there are a frequency-division multiplexing access (FDMA) system, a time-division multiplexing access (TDMA) system and a code-division multiplexing access (CDMA) system.
The FDMA system is such a communication method in which each modulation wave modulates a separate sub-carrier wave having its frequency separated a certain width. That is, in FDMA, signals occupying non-overlapping frequency ranges are summed together. By using different frequency bands, two or more separate signals can be transmitted by one and the same transmission channel. A desired signal can be taken out by a filter. This multiplexing system is not in need of synchronization.
The TDMA is a communication system in which a transmission device uses the common channel intermittently and a channel is established in a specified receiver device by an automatic distribution function. Specifically, signals compressed to high-speed burst signals are arranged in specified time slots in such a manner as to evade temporal overlap. The desired signal is reproduced on extracting the time slots. The system is synchronized because timing reference is required.
The CDMA is a multiplexing communication method employing insignia (identifiable properties or codes) proper to the signals. Demultiplexing is by utilizing code correlation characteristics with previously known reference signals. The signals handled with this system are usually digital signals.
If, in the above-described FDMA, TDMA or CDMA, the information is multiplexed, there is needed a band broader than the bandwidth of the original signals. If it is attempted with these systems to transmit 4-channel signals, for example, with 32 kbps, a band of 32 kbps is required, thus leading to an extremely high transmission rate.
In the conventional practice, if it is attempted to transmit the information simultaneously within one and the same band from the same site to some other same site, the band needs to be enlarged as compared to the bandwidth of the original information. Thus, if the bandwidth is limited, the number of channels that can be accommodated is restricted.